The invention relates to a cross-linked phenolic resole. This resole demonstrates a high carbon yield when used as a refractory binder and it exhibits high temperature and oxidation resistance plus the enhancement of thermal shock resistance.
Among the many uses for phenolic resins is their use in the production of refractory materials. Refractory materials are generally made of basic aggregates and phenolic resins as binders. The aggregates are mixed with the resins and then pressed or molded into a desired shape, i.e., brick, castables and so forth. The aggregate may also be used to make unshaped refractories. Particular refractories, such as refractory bricks, are used to line the inside wall of high temperature kilns or furnaces. The resins used for this purpose are usually thermosetting or may be cured at room temperature by the addition of a catalyst.
Phenol-formaldehyde resole resins that cure at ambient temperatures may have unacceptable thermal properties. For example, they may crack or tear when subject to the high temperatures of a kiln or furnace. The resins should optimally contribute carbon when subject to the temperatures of a kiln or furnace. The carbon contributes to the dimensional stability of the refractory articles via refractory bonding and high thermal conductivity and provides abrasion resistance. As one example, room temperature cured refractory binder comprising a novolac, a resole, and an ester as the curative is reported to improve carbonization of the resin binder and have improved thermal properties.
Refractory compositions, when molded and fired, may not have satisfactory flexural strength. The strength of refractories can be improved if the phenolic resin binder is added in large quantities during the production of the refractory material. However, the addition of large quantities of phenolic resin binder increases the cost of the refractory material, and the excessive phenolic resin in refractories generates a large quantity of hydrogen gas on decomposition which takes place when the refractories come into contact with molten metal. In the case of cast iron, the hydrogen enters the molten metal and stays in the free form, causing cold brittle fractures of the cast iron. Improved flexural strength has been reportedly accomplished by mixing a basic aggregate with a phenolic resin and a compound having a pKa value of less than 9.5 at 25xc2x0 C., particularly the phenolic resin has been incorporated with methoxymethylphenyl ether.
In an attempt to increase strength, improve cure rates, and prevent melting, amine groups have been incorporated into resins. For example, binders comprising a novolac, and amino-compound, a solvent and water have been useful in binding coked dolomite. The novolac solids are dissolved in a triethylamine (xe2x80x9cTEAxe2x80x9d)/furfuryl alcohol composition. The amino-compound used contained one to five amine groups, preferably two to four. Bricks made from the doloma aggregate mixed with the binder solution are reported to show good ambient temperature green strength and enhanced modulus of rupture after curing and coking.
Particulate resoles useful as refractory binders have been prepared by reacting phenol and formaldehyde with an amine, such as hexamethylenetetramine (xe2x80x9cHMTAxe2x80x9d), in an aqueous medium containing a colloid. The particulate resole may then be further reacted with an alkaline compound to convert hydroxylic groups to phenate groups. This second step may be a pH dependent equilibrium reaction between the alkaline compound and the phenolic hydroxyl groups. The resins recovered from these processes have exhibited increased cure rates and increased agglomeration without melting. Unlike the present invention where the goal is to pyrolize the carbon, the sintering process of this prior art was to agglomerate the particles without melting them.
High-carbon-yield refractory binders comprising a liquid resole phenolic resin and a solvent, admixed with a tar, pitch or mixture of tar and pitch are known. The resin is prepared using a catalyst system composed of ammonia, amines or a mixture of alkali metal oxides, hydroxides, carbonates in combination with ammonia or amines. The fixed carbon content resulting from use of these binders is greater than the level of fixed carbon content obtained from either the resin or from tar/pitch used individually.
Accordingly, there is a need for a high carbon yield resin useful as a binder in refractory materials and that increases refractory strength and thermal conductivity. There is further a need for a high carbon yield resin that is economical to make and use. And further a need to simplify the process by which such resins are produced. There is a need for a refractory material having these properties that does not also require large quantities of phenolic resin binder which increases the cost of the refractory in order to provide the required strength of the material.
The present invention provides a high carbon yield phenolic resole for use in the manufacture of refractory materials. The high carbon yield phenolic resole is a pre-cross linked, pre-cured resole which comprises a liquid resole having a phenol to formaldehyde mole ratio (P/F) ranging from about 1/0.5 to about 1/3.5. Preferably, the P/F ratio is about 1/0.8 to about 1/1.5. Hexamethylenetetramine may be used as a cross-linking agent in concentrations of about 2 percent to 20 percent based on the weight of the liquid resole to provide the required pre-cure.
The high carbon yield phenolic resole of the present invention demonstrates a surprising and unexpected carbon yield of about 70 percent, as compared to about 50percent to 55 percent when conventional resoles are used, at 1000xc2x0 C. under nitrogen. The unexpected higher carbon yield is coupled to a complementary unexpected decrease in volatile components. The advantage of the resin of the present invention lies in the fact that it yields significantly fewer volatile components when exposed to elevated temperatures. The lower concentration of volatile components makes this resin particularly suited for use in binding refractory materials.
The resoles described herein are the products of a controlled pre-curing process that increases carbon yield and have not been previously described. These resoles also exhibit an increase in carbon yield that is not predicted by the particular continuum of growth the resoles should adhere to throughout the curing process. These resoles exhibit physical properties that make them particularly suited for use in refractory materials.
According to one embodiment of the present invention there is provided a high-carbon-yield resole obtained by reacting a resole and a cross-linking agent such as HTMA. In further embodiments of the invention the high-carbon-yield resole includes a resole reacted with a cross-linking agent such as HTMA and dissolved in solvents such as DBE-2, furfuryl alcohol, or furfural.
Resole resins are thermosetting, i.e., they form an infusible three-dimensional polymer upon application of heat and are produced by the reaction of a phenol and a molar excess of a phenol-reactive aldehyde typically in the presence of an alkali, alkaline earth, or other metal compound as a condensing catalyst.
The phenolic resole which may be used with the embodiments of the present invention may be obtained by the reaction of a phenol, such as phenol itself, cresol, resorcinol, 3,5-xylenol, bisphenol-A, other substituted phenols, and mixtures of any of these compounds, with an aldehyde such as, for example, formaldehyde, paraformnaldehyde, acetaldehyde, furfuraldehyde, and mixtures of any of these aldehydes.
A broad range of phenolic resoles in fact may be used with the various embodiments of this invention. These can be phenol-formaldehyde resoles or those where phenol is partially or completely substituted by one or more reactive phenolic compounds and the aldehyde portion can be partially or wholly replaced by other aldehyde compounds. The preferred phenolic resole resin is the condensation product of phenol and formaldehyde.
A molar excess of aldehyde per mole of phenol is used to make the resole resins used in the present inventions. Preferably the molar ratio of phenol to aldehyde is in the range of from about 1:0.5 to about 1:3.5, and more preferably from about 1:0.8 to 1:1.5. A convenient way to carry out the reaction is by heating the mixture under reflux at atmospheric or reduced pressure conditions. Reflux, however, is not required.
The reaction mixture, is typically heated until from about 80percent to about 98 percent of the aldehyde has reacted. Although the reaction can be carried out under reflux until about 98 percent of the aldehyde has reacted, prolonged heating is required and it is preferred to continue the heating only until about 80 percent to 90 percent of the aldehyde has reacted. At this point, the reaction mixture is heated under vacuum at a pressure of about 50 mm of Hg until the free formaldehyde in the mixture is less than about 1 percent. Preferably, the reaction is carried out at 95xc2x0 C. until the free formaldehyde is less than about 0.1 percent by weight of the mixture. The catalyst may be precipitated from the reaction mixture before the vacuum heating step if desired.
The preferred phenolic resole used here is a liquid resole having the phenol and formaldehyde reacted to an endpoint by a condensation reaction. The resole is then further reacted by cross-linking with HTMA in a concentration of 2 percent to 20 percent based on the weight of the resole and preferably at a concentration of 8 percent to 12 percent based on the weight of the resole.
The high-carbon-yield resoles were prepared by adding the liquid resole and HTMA at room temperature and then heating the combination to a temperature of 80xc2x0 C. The HTMA may also be added after the liquid resole resin has been heated. The combination of liquid resole resin and HTMA is held at an elevated temperature for a period of time between 5 and 30 minutes and then cooled.
Diluents may be added to the liquid resole/HTMA combination or to the pre-cured resole. Diluents may include furfuryl alcohol, DBE-2 dibasic ester, furfural, or others. Examples of pre-cured resoles of the present invention are provided below. The resoles of the examples are characterized in Table 1.